As a method for manufacturing cellulose ethers, after cellulose is converted to alkali cellulose in the presence of water and a base, an etherifying agent is reacted with the alkali cellulose, which is the method now in general use (a method falling in this category is referred to as the general method hereinafter). In the general method, as an etherifying agent is consumed by water contained in an alkali cellulose, etherification of cellulose is often hindered or the etherification reaction often proceeds in a non-uniform system, so that it is impossible to manufacture cellulose ether having a high degree of substitution by a one step reaction.
As the general method carried out with the aim of manufacturing of cellulose ethers having a high degree of substitution or of improvement in the reaction efficiency of an etherifying agent, there can be cited (a) a multi-stage reaction method for reetherifying cellulose until an object etherification degree is obtained, (b) a multi-stage alkali addition method [Japanese Patent Laid-Open No. 45201/1983], (c) a method for combining multi-stage reactions with multi-stage alkali addition [Japanese Patent Laid-Open No. 176202/1983], (d) a method for adding additives such as a dispersion assistant and a catalyst transferring between phases to a reaction system[Japanese Patent Laid-Open No. 103501/1983], [W. H. Daly, J. D. Caldwell: Polymer Sci.:Polymer Letters Ed.,17, 55(1979)] and the like. The method (a) is a general method for manufacturing cellulose ethers having a high degree of sunstitution, but it is not an advantageous that the etherification reaction is carried out many times and the manufacturing cost is increased in it. Although the limit of substitution degree for carboxymethylation reaction by a usual method was 1.7-1.8, the substitution degree of 2.2 was attained by the method(b) using the same amount of reagent as the above, but a substitution degree exceeding 2.2 cannot be expected by the one stage reaction. As the method(c), carboxymethylation was carried out two times by the method(b) and CMC having a degree of substitution of 2.95 was obtained. Therefore, the method(c) is fairly more advantageous than the method(a) alone. In the method(d), it was disclosed that use of a quaternary ammonium salt as a catalyst transferring between phases for synthesis of benzyl cellulose resulted in a rapid progress of the benzylation reaction. However, the substitution degree of the resulting benzyl cellulose was 2.4 at utmost. Several methods and proposals as the general method have been presented so far, but it is yet difficult to manufacture cellulose ether having a degree of substitution of 2.5 and over by the one stage reaction.
As a cellulose ether having a high degree of substitution which is now available industrially, there can be cited ethyl cellulose having a degree of substitution of 2.5. However, cellulose ethers other than that are not manufactured industrially, so that their characteristics and applications are hardly opened up. Cellulose derivatives have various functions according to properties of substituent groups and, in addition, the structure of cellulose has hidden potentiality of imparting a high degree function to cellulose. For example, benzyl cellulose is known to have a softening point and also excellent electric characteristics, and recently it has been found that tribenzyl cellulose (having a degree of substitution of 3) has an excellent optically resolving ability for the optical isomer of physiological active compounds such as medicines, agricultural chemicals and foods[Japanese Patent Application No. 226527/1983]. Further, formation of liquid crystal as found with a cellulose derivative can be cited as one example of the said high degree function. On the other hand, cellulose derivatives having a substituent group with an unsaturated bond at the side chain have a high utilizing value as reactive cellulose derivatives. For example, cellulose allyl ether can be grafted easily with styrene or the like, and as cinnamyl cellulose has properties that it becomes insoluble to a solvent by exposure to light or heat, it can be expected to apply it to a heat curing coating or to a photoresist. However, when benzyl cellulose and cinnamyl cellulose are manufactured by the general method, that is, a method for reacting an etherifying agent with alkali cellulose, the etherification reaction does not proceed uniformly, so that the cellulose ether having good solublity in a solvent or a high degree of substitution is hard to prepare. That seems to be because the surface of cellulose fiber etherified at the beginning time of etherification reaction becomes a hydrophobic layer which cannot be swelled by the aqueous alkali solution and consequently the etherifying agent cannot penetrate into the inside of cellulose.
As the method for preparing benzyl cellulose having a high degree of substitution by a method other than the general method, there can be cited a method for using a solvent-soluble cellulose derivative as the raw material and a method for using a special solvent for dissolving cellulose. These methods are ones for attempting to solve difficulties of etherification reaction and non-uniformity in the same reaction caused by the shape of cellulose fiber by dissolving the raw material to prepare a uniform solution. As the method for using a cellulose derivative as the raw material, there can be cited a method for benzylating cellulose acetate by Hakomori's method[S. Hakomori: J. Biochem. (Tokyo), 35,205-208(1964)]. This is the benzylating method by dissolving cellulose monoacetate(having a degree of substitution of 1) as the raw material in dimethyl sulfoxide and then using dimsyl ions (CH.sub.3 SOCH.sub.2.sup.-) and benzyl chloride to benzylate the cellulose acetate[G.Keilich, N.Frank and E.Husemann: Makromol. Chem., 176, 3269(1975)]. Although benzyl cellulose having a degree of substitution of 2.95 was obtained by the method, its yield was as low as 30%. In the method for using a solvent for cellulose, after cellulose was dissolved in a solvent system consisting of SO.sub.2, diethylamine and dimethyl sulfoxide, powdered sodium hydroxide was added to the solution and then divided addition of benzyl chloride was carried out for benzylation (A. Ishizu, A. Isogai, T. Ishii and J. Nakano: Paper contributed for International Symposium on Wood and Pulping Chemistry, Vol.1, 70(1983)). Benzyl cellulose having a degree of substitution of 3.0 was obtained at a high yield by the method. However, the method required a special solvent for dissolving cellulose and also the operation for dissolving cellulose was troublesome. In the method, when powdered sodium hydroxide was added to the cellulose solution, cellulose became an insoluble compound, which precipitated in the form of granule, so that the reaction system became once a non-uniform system. Therefore, as the benzylation reaction proceeded, the reaction product was dissolved in the solvent. However, completion of the reaction and formation of a uniform system took a long time. In the above-mentioned methods, the benzylation reaction was carried out under condition of considerably few amount of water as compared with the general method and also a good solvent for both raw cellulose and benzyl cellulose was used, so that uniformity of the reaction was improved and the benzyl cellulose product had a high degree of substitution. However, the methods had the defect such as a low yield or too long a reaction time( of 16-20 hours).